This invention relates to a method of polishing the target surface of a tape-shaped base material (or “substrate”) for forming an oxide superconductor thin film on such a polished surface, as well as a base material for an oxide superconductor using such a tape-shaped substrate, which has been polished by the method of this invention.
Among superconducting materials, it is known that oxide superconductors are superior superconductors having critical temperatures higher than the liquid nitrogen temperatures. As disclosed in Japanese Patent Publication Tokkai 9-120719, for example, a tape-shaped material obtained by forming a polycrystalline directional film of MgO, yttrium stabilized-zirconia or CeO2 having controlled crystalline directionality as an intermediate layer by an IBAD (ion beam assisted deposition) or PLD (pulsed laser deposition) method on the surface of a Hastelloy alloy tape comprising a nickel alloy and further forming a YBCO (YBa2Cu3O7-y) oxide superconductor film on this polycrystalline directional film is known as a typical example of oxide superconductor tape-shaped material. In order to provide an oxide superconductor of this kind for a practical use, however, there are still many problems currently remaining to be solved.
As disclosed in Japanese Patent Publications Tokkai 2-207415, 6-145977 and 2003-036742, for example, there have been methods of making the surface of a tape-shaped substrate smoother in order to obtain a higher critical current Ic and a higher critical current density Jc such that a superconductive film with a superior crystalline characteristic can be formed on a tape-shaped substrate.
Moreover, if the directionality of the intermediate layer is improved, the directionality of the superconducting film that is formed thereupon is also improved. In particular, it is considered indispensable to obtain a high biaxial directionality in order to obtain a superconducting film having high critical current Ic and high critical current density Jc.
Since the crystalline characteristic of the intermediate layer to be formed depends on the crystalline characteristic of the surface of the tape-shaped substrate which serves as its base, the crystalline directionality and the in-plane directionality of the tape-shaped substrate become important in order to obtain an intermediate layer with a good directionality. Thus, in order to crystallize the intermediate layer film with a good directionality, it is necessary to finish the surface of the tape-shaped substrate with flatness and smoothness at the nanolevel.
U.S. Pat. No. 6,908,362 has disclosed another method according to which the surface of a tape of nickel or a nickel alloy is precisely polished and thereafter an oxide superconductor film is formed.
Another problem that has been preventing superconductivity from becoming practically usable relates to the behavior of the quantum of magnetic flux. The Lorentz force generated by a current that passes through a superconductor tends to move the magnetic flux quantum but if the magnetic flux quantum moves, heat is generated and breaks down the condition of superconductivity. Thus, the critical current Ic that can be passed through a superconductor can be made larger if the magnetic flux quantum can be pinned down so as not to move.
These prior art technologies described above all indicate the importance of polishing the substrate surface to make it flat and smooth.
A tape-shaped material is usually formed by drawing a metallic material into an elongated tape-like shape with a thickness of 0.05 mm-0.2 mm while repeating rolling and heating processes. On the surface of such an elongated material, there are mechanical linear marks that are formed due to the rolling and changes due to crystalline defects. These linear marks and defects tend to adversely affect the crystalline directionality of the intermediate layer and the superconducting layer that are directly formed thereupon. For this reason, it has been a common practice in the manufacturing of tape-shaped superconductor materials to initially form a flat and smooth surface on the tape-shaped material after the rolling process by mechanical polishing or electrolytic polishing and then to form an intermediate layer and a superconducting layer thereupon, as disclosed, for example, in Japanese Patent Publications Tokkai 6-31604 and 2002-150855.
In the case of a very long tape-shaped substrate, however, it is extremely difficult to polish it to make it flat and smooth over its entire length.